The very first addon people think about when they think of making money on the AH is Auctioneer. This addon is a complete replacement for your AH UI, and it has an absolute ton of features. I remember setting it up the first time I decided to try my hand at gold making; it was one of the most complicated addons I'd ever seen. It felt like 90% of the things I could fiddle with in the options weren't connected to what I was trying to do, but I kept working through it hoping I'd find what I was looking for. In the end, I settled for preventing it from directly causing me to lose money and assumed that I was missing out on some serious profits because I wasn't able to set Auctioneer up properly.
I've since learned where all the actually useful options are located and how to use them, but I've already on to other addons that provide the same options with less bloat. Auctioneer is a powerful tool, but it suffers from the kind of bloat you get when you put a bunch of programmers into a room and let them run wild. It's still got some features that aren't executed as well by competitors (their snatch list for buying comes to mind), but it doesn't have enough compelling features that I'd recommend it to people getting started.
Auctionator is an addon that I wish I had had available when I started. It has a pretty intuitive, simple buy and sell interface, and (most importantly) it groups all similar auctions together. You can see at a glimpse how many items are posted at what price and stack size, to base your buying and selling decisions on that. It also natively supports saved searches with categories. In short, it does everything the default UI should do. Both power users and retail auctioneers can benefit from it, as it's simple enough for anyone to use and powerful enough that everyone should. Don't subject yourself to the default buying interface any longer than you have to.
It's not perfect, though. In my opinion, Auctionator could put the final nail in the coffin of Auctioneer if it'd replicate the snatch list, allowing people to create systematic buying schemes that can help save time for serious buyers. Still, what it does it does well.
I've talked about TradeSkillMaster a lot lately and written a few guides to get you started if you're interested in learning it. TSM is not a simple addon and by no means a simpler solution to simple crafting and selling, but if you find yourself buying, crafting, and selling the same things again and again, using it will save you time.
Understand the limitations of this addon, though. It's not designed to make crafting or selling easier the first time. It is not a complete solution, and you need to be able to buy, sell, and craft outside the algorithmic setup you've created in TSM to remain agile and competitive.
4. Advanced Trade Skill Window
ATSW is a replacement for the default profession interface. It supports a queue, it's twice as wide and much easier to read, and it allows you to build a custom interface precisely how you want it. It's fairly user-friendly out of the box and supports a lot of customization. This makes it a winner, in my book.
It's particularly good at queuing, by the way. If you tell it to make something that has subcomponents you have to craft, it will queue them up for you. TSM only started doing that at the last release. It supports auto-buying of vendor mats and is basically like the crafting arm of TSM, only without all the other complexities added to it.
My only criticism of ATSW is that since Blizzard made the default profession UI actually usable in patch 4.0, the only value add features is has are also done by other addons like TSM. It'll always have a niche market of people who don't want to learn a tool like TSM but want craft queuing. The rare times I've needed to go outside TSM's automation to craft things, I've found the default UI good enough. If I have to do something enough that I'd need a queue, I add it to TSM.
5. LilSparky's Workshop
LSW is an addon that puts auction prices next to items you can craft in your profession window, as well as the cost to craft them. It's supported with ATSW, so you can use them together. It's not needed if you use TSM, since this is a baseline feature of that addon -- but again, if you're not going to use it and still want the information put together nicely, this'll do it. It also works with the default profession window.
Lil Sparky also has another project he's working on called Gnomeworks, but it's still in a semi-working beta state.
Clear as mud
Addons don't need to be complex to work. Whether you go whole hog and set up something like TSM, or keep it simple and just your UI so that it works the way the basic interface should, you will benefit by installing them.
Maximize your profits with more advice from Gold Capped as well as the author's Call to Auction podcast. Do you have questions about selling, reselling and building your financial empire on the auction house? Basil is taking your questions at firstname.lastname@example.org.